Snowpack stabilizes, but more snow headed to Sierra Nevada

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After two avalanches last week, the Tahoe snowpack has started to stabilize according to Tahoe National Forest Sierra Avalanche Center forecasters.

But that trend could reverse if the fast-moving storm predicted to hit the South Shore Tuesday brings another foot or more of fresh snow.

The Sierra Avalanche Center, a nonprofit that provides daily avalanche advisories during the winter, recorded two human-triggered slides last week. A skier triggered a 700-foot-long avalanche on March 7 around 4 p.m. near Donner Summit, while a snowboarder caused another slide March 9 at Mount Rose that temporarily reduced traffic to one lane on the Mount Rose Highway.

Although both the skier and rider were caught in the avalanches, Sierra Avalanche Forecaster Brandon Schwartz said he was unaware of any injuries.

Both the slides failed on a weak layer in the snowpack that formed under the weight of last week’s heavy snow, Schwartz said.

The snow that fell around March 6 sat on a hard crust formed at higher elevations by a melt-freeze cycle and at lower elevations by a mix of rain and wet snow, Schwartz said. Imagine a dense, wet layer of slush separated from a loose, sugary mix by an icy crust and you’ll have some idea of the regional snowpack.

It’s a volatile combination that is only now starting to stabilize, according to Schwartz.

The record-high temperatures from the past few days helped settle the snowpack, but Schwartz said that trend could reverse if another system rolls through the basin.

“There’s some stabilization of that persistent weak layer, but once we add new snow, it’s likely to turn around,” Schwartz said Thursday.

“Cold air will begin to filter in as the cold front approaches Tuesday night and snow levels should drop to lake level by Wednesday morning. The heaviest precipitation will fall during the day on Wednesday as the jet stream takes direct aim at us. This will be a decent snowstorm all the way down to lake level,” Allegretto posted on opensnow.com/dailysnow/tahoe. “This storm could drop as much snow as the last storm but in one day instead of 3 days.”

A Reno area forecast discussion from the National Oceanic and Atmospheric Administration also predicted a high chance of precipitation starting Tuesday night and moving into Wednesday.

Whether a similar crust forms under the anticipated new snow waits to be determined, Schwartz said. Wet slush from 60-degree days that refroze at night could create another weak layer, but forecasters won’t know until it happens, he said.